1. Field of the Invention
The present invention relates to a spread spectrum wireless communications device used in a cellular phone, a portable telephone system, a cordless telephone system, or a wireless LAN system and, more particularly, to a spread spectrum wireless communications device enabling code division multiple access (CDMA) communications using a spread spectrum method.
2. Description of the Related Art
In recent years, spread spectrum techniques that are robust against interferences and jamming have attracted attention as one wireless communications method used in mobile communications systems. Portable telephones according to U.S. standard IS-95 have started to be used in Korea, in USA as well.
In the USA where such services are provided in practice, the frequency of the base station differs according to each individual service provider. Where a hard handoff of a mobile station occurs between base stations using such different frequencies, dropped calls take place frequently, thus presenting a problem.
Referring to FIG. 14, a first service provider uses a base station BS1. A mobile station MS using a frequency f1 is connected with the base station BS1. A second service provider has a base station BS2 using a frequency f2 different from the frequency f1. If the mobile station MS moves to a cell of the base station BS2 of the second service provider, a hard handoff occurs without actually searching for the phase of the PN code of the signal from the base station BS2 by a searcher. Therefore, if a base station with a PN offset carried by a message sent from the base station BS1 is not available, or if the PN code shifts by more than 0.5 chip by the effects of the wireless path, dropped calls occur.
Accordingly, the U.S. Telecommunications Industries Association (TIA) recommend that a pilot search of base stations using different frequencies be conducted during a conversation (traffic channel), which constitutes improvement of an IS-95 system. However, if such a pilot search is carried out, the reception frequency must be changed to a frequency different from the frequency of the traffic channel. Consequently, during time intervals in which the reception frequency has been modified in this way, it is impossible to receive the frequency of the current conversation.
With the IS-95 method, the chip rate is 1.2288 MHz. It is assumed that the window length searched by a searcher at a different frequency given from the base station is xc2x1120 chips and that the searcher has only one complex correlator. If integration is performed over 16 symbols per one PN code phase, the time required for the pilot search is 16 symbolsxc3x9764 chipsxc3x97240 chips (window length)÷1.2288 M=200 msec
Since 1 frame is 20 msec, the above-described time corresponds to 8 frames. It follows that the voice of the conversation of the current traffic channel cannot be received during the periods of 8 successive frames. In the calculation described above, it is assumed that the searcher has only one complex correlator. If it has four complex correlators, the time corresponds to 8÷4=2.5 frames. Eventually, three consecutive frames cannot be received.
FIG. 15 illustrates this situation, i.e., a pilot search is carried out over 3 frames. In this FIG., A(A1, A2) is an active set and represents the PN offset of a presently busy channel (T-channel). C(C1, C2, C3) is a candidate set and indicates the PN offsets of base stations that are not busy but have reception levels greater than a threshold value. N(N1) is a neighboring set and indicates the PN offsets of base stations that are included in a neighboring list and have reception levels not exceeding the threshold value. R(R1, R2) is a remaining set and indicates the PN offsets of other base stations.
In the configuration shown in FIG. 15, the active setA(A1, A2) of the frequency f1 is first searched. Then, the candidate set C(C1, C2) is searched for pilots. Subsequently, the reception frequency is switched to f2, and the neighboring set N1 is searched. If this state persists during the period of 3 frames and the search of the neighboring set N1 ends, the reception frequency is returned to f1. In this way, the frequency of the current conversation cannot be received over the periods of 3 frames. In consequence, the voice speech is interrupted.
Where the mobile station is at the boundary between the coverage areas of adjacent base stations, it may be necessary to search numerous candidate sets for pilots. In the past, individual candidate sets are searched in succession. Hence, it takes a long time to complete the search of all the candidate sets for pilots.
As described thus far, in the prior art search of neighboring sets for pilots, voice is interrupted over 3 frames, resulting in auditory unnaturalness. Also, in search of numerous candidate sets for pilots, it takes a long time to complete the search.
In view of the foregoing circumstances, the present invention has been made. It is a first object of the present invention to provide a spread spectrum wireless communications device capable of conducting a pilot search for frequencies different from a traffic channel frequency during a conversation while suppressing deterioration of voice quality.
It is a second object of the invention to provide a spread spectrum wireless communications device capable of conducting a pilot search of numerous candidate sets in a short time.
The object described above is achieved in accordance with the teachings of the invention by a spread spectrum wireless communications device used in a spread spectrum mobile communications system having base stations using different frequencies, said wireless communications device comprising:
receiver means for receiving first signals transmitted over a first frequency used by a first base station;
reception frequency control means for controlling said receiver means so as to receive second signals transmitted over a second frequency used by a second base station during periods of discretely selected frames; and
phase search means for searching pseudorandom code phases of the received second signals in response to the reception frequency control means.